--- title: "Grouping Species Using DNA" description: "Use shared derived traits, including anatomical and DNA evidence, to explain cladistics. Compare corresponding DNA sequences and introduce calibrated molecular clocks. Explain birds' placement within " canonical: https://lightmysky.com/learn/science/grouping-species-using-dna-mt_fqAkSv3cUE source: https://lightmysky.com/learn/science/grouping-species-using-dna-mt_fqAkSv3cUE.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Grouping Species Using DNA Use shared derived traits, including anatomical and DNA evidence, to explain cladistics. Compare corresponding DNA sequences and introduce calibrated molecular clocks. Explain birds' placement within theropod dinosaurs using fossil and skeletal evidence. Compare convergent and parallel evolution with stated conventions. Describe horizontal gene transfer and revisions to classification as evidence changes. Subject: Science · Area: Animals of the World · Ages 13 to 14 Page: https://lightmysky.com/learn/science/grouping-species-using-dna-mt_fqAkSv3cUE ## Lesson: Testing species relationships You have compared traits that help survival. To test ancestry, ask whether similar traits were inherited from a shared ancestor or evolved separately. Cladistics tests ancestry using inherited shared derived characters. A derived character changes an ancestral condition; losing a feature can count too. Evidence can come from body features, fossils and molecules, including DNA. A phylogenetic tree is a testable hypothesis about evolutionary relationships. **Example.** Crocodiles resemble lizards, but share a more recent common ancestor with birds than with lizards. Crocodiles and birds belong to archosaurs. Lizards belong to a different reptile branch. Anatomical evidence and DNA comparisons support this relationship. Scientists align corresponding DNA stretches inherited from common ancestors. Compare the letters at matching positions and count differences. With three sequences, compare each pair separately. Equal-length stretches changing at comparable rates can suggest relative split times, but a few letters alone cannot establish a real species tree. *(drawing: Each cell compares one corresponding position. Only C / A differs, giving one difference.)* A molecular clock connects DNA changes to estimated time using change rates and dated evidence, such as fossils. Rates vary among genes and lineages. Under a comparable-rate model, 4 differences suggest less elapsed time than 7 in equal-length stretches. Counts alone do not give exact dates. Birds evolved within theropod dinosaurs. Fossils and inherited skeletal features support this placement without requiring DNA from Tyrannosaurus. A clade includes an ancestor and all its descendants, so birds remain dinosaurs. The same rule keeps whales within mammals. Convergent evolution produces similar features independently in different lineages. Bird and bat flight wings evolved separately, although their forelimb bones share older ancestry. Cacti and some euphorbias independently evolved thick, water-storing stems. Independent evolution concerns the particular feature, not a lack of all common ancestry. Parallel evolution often describes independent similar changes from similar inherited starting mechanisms. Here, two skink lineages independently reduce pigment through changes in the same inherited pigment pathway. We call this parallel evolution. Relatedness alone does not decide the label, and scientific usage varies. Horizontal gene transfer moves DNA between organisms outside parent-to-offspring inheritance. It occurs within or between species, is common in bacteria and also occurs in other groups. A transferred gene can have a different history from other genes. Networks can show transfer without erasing ancestry. Matching DNA alone does not prove transfer. Scientists revise classifications when tested evidence supports a different relationship. One matching gene does not settle every branch. When genes disagree, compare more genes and check other evidence before deciding. The newest claim is not automatically the best-supported explanation. **Recap.** DNA, fossils and body features test ancestry. Birds remain dinosaurs. Independently evolved similarities, changing DNA rates and gene transfer can complicate comparisons. *Lesson, questions and flashcards written for this dataset.* ## Practice 24 questions on this page, each with its working shown. ## Needs first - [Chromosomes, Genes & DNA](https://lightmysky.com/learn/science/chromosomes-genes-and-dna-mt_6aJUzBYGNs) ## Optional connections - [Evidence-Based Classification](https://lightmysky.com/learn/science/evidence-based-classification-mt_B8lX8OCzGu) ## Opens up - [Taxonomy, the three domains and what a species name encodes](https://lightmysky.com/learn/science/taxonomy-the-three-domains-and-what-a-species-name-encodes-mt_12ucjuHtHE) - [Speciation and the Rate of Evolution](https://lightmysky.com/learn/science/speciation-and-the-rate-of-evolution-mt_OdcdfeRyo1)